While the general diet of Mediterranean elasmobranchs has been widely studied, little is known about food partitioning and competition among sympatric species, despite these being important forces ...structuring marine communities. Using stomach content and stable isotope analyses, we investigated diet and trophic levels and evaluated the diet overlap and partitioning of Scyliorhinus canicula, Mustelus mustelus, and M. punctulatus in the northwestern Adriatic Sea. These shark species were confirmed as opportunistic mesopredators, but significant differences in their diets emerged. The two bentho-demersal Mustelus species had a larger trophic overlap with S. canicula than between each other. Given the pronounced morphological similarity of these two Mustelus species, this is likely a strategy to limit competition. The strictly benthic S. canicula showed a more varied diet compared to the other species. Stable isotope analysis highlighted that despite the smaller size and overlapping diets, S. canicula occupied a slightly higher trophic level. A better characterization of the trophic role of these species in the food web of the basin can be obtained from these data. At an ecosystem level, this information is essential to evaluate the possible consequences of the decline or recovery of the population of these exploited species.
For animals discarded after fishing capture (unwanted bycatch), the effects of fishing-capture stress can extend beyond immediate or delayed death, causing long-term, sub-lethal effects such as ...injuries and physiological, behavioral, immune, energetic, and reproductive consequences. Given the importance of successful reproduction for population recruitment, investigating reproductive impairment is essential, even more so for animals with low reproductive rates such as chondrichthyans. However, data for these species are lacking, even though the poor conservation status of many chondrichthyan populations and the threats from overexploitation and discard require a better understanding of fishing-capture stress consequences. In this study, we investigated the reproductive consequences of trawling and air exposure stress when experienced by southern fiddler rays (
Trygonorrhina dumerilii
) during late pregnancy, with a focus on neonatal traits. Compared with neonates from unstressed mothers, neonates born from mothers subjected to capture simulation had significantly lower body mass and yolk sac volume at birth, showed a granulocyte-to-lymphocyte ratio indicative of a stressed condition, reduced growth, altered burying behavior, reduced boldness, and swam for shorter distances after a simulated predator attack. Smaller size and lower growth rate might expose neonates to a higher rate of predation, and similarly, the altered burying and swimming behaviors may reduce their ability to escape from predators. Decreased boldness could impact neonate survival by reducing their ability to compete and obtain food and resources. Further investigations are needed, but these initial results suggest that capture stress suffered by pregnant rays may alter traits and survival of their offspring with consequences for recruitment and population abundance.
Abstract
Elasmobranch populations are declining worldwide, calling for urgent assessment of fishery exploitation and application of effective conservation strategies. Here, we applied a novel ...approach, integrating long-term time-series of landings (1945–2012) and extensive surveys at the fish market of Chioggia, Italy, home of the major fishing fleet of the northern Adriatic Sea, to evaluate the status of elasmobranch populations and fisheries in the one of the most fished Mediterranean basins. The time-series highlight a dramatic decline in elasmobranch landings, particularly for skates and catsharks (Scyliorhinus spp.), whose current catch rates are 2.4 and 10.6% of the average 1940s levels, respectively. These data likely reflect similar large reductions in abundance, as indicated by the analysis of catch-per unit-effort time-series. The biomass of landed skates and catsharks showed regular fluctuations that disappeared after the collapse of the landings. Elasmobranch market composition, assessed through the sampling of 11 900 specimens from 2006 to 2013, included 14 species, but was dominated by just two: Mustelus mustelus and M. punctulatus, which represented more than 60% of the catch. The proportion of sexually immature individuals was generally very high, up to 83% of landed females and 71% of landed males, depending on the species. Although some correlations were detected between landings and local hydrography or climatic indices, the analyses of landings and surveys at the fish market identified fishery exploitation as the main driver of the striking, long-term elasmobranch decline in the northern Adriatic Sea, calling for urgent management actions to improve the conservation status of these fish.
Elasmobranchs are among the most threatened long-lived marine species in the Mediterranean Sea. Incidental catch and intense fishing effort are considered the major threats to their conservation. To ...date, no Mediterranean countries has adopted specific technical measures within current conservation plans to mitigate potential impacts on vulnerable elasmobranchs. This study aimed to evaluate the effectiveness of new elasmobranch bycatch reducing devices (BRD) in a Mediterranean multispecies bottom trawl fishery (northern central Adriatic Sea, one of the most exploited basins in the Mediterranean). Specifically, two types of grids were tested: a novel shark excluder device (SED) and a flexible grid (Flexgrid). Effectiveness was evaluated in terms of bycatch and commercial catch reduction, relying on the different sizes of elasmobranchs and the main commercial species. Results showed that preliminary tests with SED were effective in excluding elasmobranch species. Catches made with this grid showed no significant difference for targeted crustacean (mantis shrimp, Squilla mantis), however, commercial losses were shown for some large bony fish. Flexgrid did not affect the catches of the main target species, such as the red mullet (Mullus barbatus), European hake (Merluccius merluccius) and mantis shrimp, but also retained small-spotted catshark (Scyliorhinus canicula). The effectiveness of the grids apparently was affected by the small size of the elasmobranchs captured. The study showed that SED and Flexgrid are valuable solutions for elasmobranchs if combined with other conservation measures (e.g. seasonal fishing closure in critical areas and good practices). Finally, strong cooperation between fishers, scientists, and managers is critical to identify the best management options in a regulatory framework where only a few elasmobranch species are formally protected. Interestingly, the study also yielded the identification of two potential aggregation-nursery areas for the small-spotted catshark (Scyliorhinus canicula) and the common smooth-hound (Mustelus mustelus).
•Excluder devices could be promising solutions to avoid elasmobranch bycatch in a Mediterranean bottom trawl fisheryo•Catches with SED showed no significant differences for targeted crustacean, with the exception of some bony fishes•Flexgrid did not affect the catches of the main target species but also retained small elasmobranchs.•The effectiveness of the grids apparently was affected by the small size of the elasmobranchs captured.•A combination of different conservation measures and a strong cooperation between stakeholders is urgently needed.
The role of natural and sexual selection in generating variability in biological traits between populations represents an intriguing issue in evolutionary biology. Considering their occurrence in ...different environments and the extensive incidence of post-copulatory sexual selection, elasmobranchs represent an interesting, yet still poorly investigated group. In this study, the life history and reproductive traits of two populations of
Scyliorhinus canicula
from the northern Adriatic Sea and the Strait of Sicily were compared. Differences in maximum size and size at sexual maturity were observed. The two populations also displayed differences in male and female genitalia. Males in the northern Adriatic Sea presented heavier testes, longer epididymis, seminal vesicles and claspers compared with those in the Strait of Sicily, suggesting the occurrence of stronger sperm competition at the former site. Similarly, females in the northern Adriatic Sea showed heavier oviducal glands and longer reproductive tracts compared with those in the Strait of Sicily. The coevolution between male and female genitalia suggests the occurrence of stronger sexual conflict and/or cryptic female choice in the population from the northern Adriatic Sea. Therefore, natural selection, represented by the different selective pressures occurring at different latitudes, and sexual selection, represented by potentially differing strengths of post-copulatory sexual selection and sexual conflict, may act individually in driving divergence in life history and reproductive traits in these two populations of
S. canicula
.
In this study we investigated the metabolic consequences of simulated gillnet capture in species with contrasting life-history characteristics, the gummy shark, Mustelus antarcticus and the Port ...Jackson shark, Heterdontus portusjacksoni. Pre-stress standard and routine metabolic rates (MR) of individuals of these species were measured and compared to the MR obtained after simulated gillnet capture. Standard MRs of M. antarcticus and H. portusjacksoni were 168.5 ± 61.9 mgO2 kg-0.67 h−1 and 144.1 ± 20.4 mgO2 kg-0.67 h−1, respectively. Routine MRs of M. antarcticus and H. portusjacksoni were 202.6 ± 63.5 mgO2 kg-0.67 h−1 and 166.4 ± 11.2 mgO2 kg-0.67 h−1, respectively. The simulated gillnet-capture treatment decreased the MR of M. antarcticus by 59.3% and increased that of H. portusjacksoni by 21.6%. Our results confirm that M. antarcticus is highly sensitive to gillnet capture, exhibiting significant variability in MR among individuals and also a high delayed mortality rate. We interpret the decrease in MR as a compensatory mechanism for reducing oxygen consumption to cope with the stress imposed by capture. Mortality in this species was associated with higher pre-stress MR, and lower post-stress MR. In contrast, H. portusjacksoni showed mild increases in MR. Their resilience to capture stress most likely relies on effective buccal pumping and lower basal MR. A mixed-effects model approach permitted us to identify the main sources of variation in the MR measured, which were the individual differences, treatment effects, species variations and delayed mortality. The comparative approach employed in this work allowed us to understand and provide reliable estimates of the effects of fishing on the MRs of these two species.
•Mustelus antarcticus is susceptible to simulated gillnet capture.•Heterdontus portusjacksoni is resilient to the effects of simulated gillnet capture.•The differences between individuals greatly contribute to total variance.•Mustelus antarcticus mortality is associated with higher pre-stress and lower post-stress MR.•Larger sharks showed resilience to changes in their MR compared to smaller sharks.
Overfishing, capture mortality, and consequences following the release of surviving animals represent severe threats to chondrichthyans. Although holocephalans are common bycaught and discarded ...species, other than postrelease mortality, little is known of fishing capture stress impacts. The stress response elicited after capture, essential to increase survival chances, is energetically demanding and affects the amount of energy available for other biological activities, with potential long‐term impairments. We measured the effect of 30‐min simulated gillnet capture on oxygen uptake rate (ṀO2), a proxy for metabolic rate and energy use, on recovery pattern, and on swimming activity of elephant fish (Callorhinchus milii). Immediately after simulated capture, Active and Inactive ṀO2, measured during swimming and resting periods, respectively, were 27.5% and 43.1% lower than precapture values. This metabolic decline is likely an adaptation for reducing the energy allocated to non‐essential activities, thus preserving it to sustain the stress response and processes essential for immediate survival. Supporting this, after gillnet capture, animals decreased their swimming time by 26.6%, probably due to a reduction in the energy allocated to movement. After 7 days, swimming activity and both Inactive ṀO2 and Active ṀO2 returned to precapture values. Although metabolic decline may enhance survival chances, the associated decreased swimming activity might increase predation risk and slow the physiological recovery after a fishing event. Moreover, some of the activities involved in Inactive ṀO2 are fundamental for life maintenance and therefore its depression after a capture event might have long‐term repercussions for life sustenance and health.
This first report of fishing‐capture induced energetic alterations in the holocephalan Callorhinchus milii shows that gillnet capture decreases swimming activity and oxygen uptake. These changes suggest health and survival impairments potentially severely affecting populations.
We develop a potentially widely applicable framework for analysing the vulnerability, resilience risk and exposure of chondrichthyan species to all types of anthropogenic stressors in the marine ...environment. The approach combines the three components of widely applied vulnerability analysis (exposure, sensitivity and adaptability) (ESA) with three components (exposure, susceptibility and productivity) (ESP) of our adaptation of productivity–susceptibility analysis (PSA). We apply our 12‐step ESA‒ESP analysis to evaluate the vulnerability (risk of a marked reduction of the population) of each of 132 chondrichthyan species in the Exclusive Economic Zone of southern Australia. The vulnerability relates to a species’ resilience to a spatial (or suitability) reduction of its habitats from exposure to up to eight climate change stressors. Vulnerability also relates to anthropogenic mortality added to natural mortality from exposure to the stressors of five types of fishing and seven other types of anthropogenic hazards. We use biological attributes as risk factors to evaluate risk related to resilience at the species or higher taxonomic level. We evaluate each species’ exposure to anthropogenic stressors by assigning it to one of six ecological groups based on its lifestyle (demersal versus pelagic) and habitat, defined by bathymetric range and substrates. We evaluate vulnerability for 11 scenarios: 2000–2006 when fishing effort peaked; 2018 following a decade of fisheries management reforms; low, medium and high standard future carbon dioxide equivalent emissions scenarios; and their six possible climate–fishing combinations. Our results demonstrate the value of refugia from fishing and how climate change exacerbates the risks from fishing.
In animals discarded after a fishing capture event, the elicited stress response necessary to ensure their survival is energetically costly. This energy is diverted from other important biological ...activities, including growth and reproduction, possibly impairing them. Given that elasmobranchs are among the most threatened vertebrate groups, estimating capture-induced energetic changes and comparing these variations to the energy requirements of pregnancy maintenance is necessary. In pregnant southern fiddler rays (Trygonorrhina dumerilii), we measured changes in oxygen uptake rate (ṀO
; a proxy for metabolic rate and energy usage) in response to trawling simulation and air exposure, and estimated the oxygen requirements of sustaining late-term pregnancy and embryos. ṀO
was measured in pregnant females, before (prestress ṀO
) and after trawling simulation (after-capture ṀO
), and again after females gave birth (postpartum ṀO
). After-capture ṀO
was 31.7% lower than ṀO
measured in minimally stressed females, suggesting a reduction in energy expenditure. This reduction is likely triggered by an initially excessive energetic investment in the stress response, and is aimed at shutting down nonessential activities to redirect energy to processes fundamental for survival. Prestress ṀO
was 78.5% higher than postpartum ṀO
. Capture simulation decreased ṀO
to values similar to those observed postpartum, suggesting a capture-induced reduction in oxygen and energy allocation to pregnancy and embryonic respiration, which could be associated with reproductive impairments. These data, by better estimating the impact of capture and discard on energetic requirements and reproductive fitness, may support the introduction of area and/or seasonal closures to fishing.
The common thresher shark, Alopias vulpinus, has shown a marked decline in abundance in the Mediterranean Sea. Through the analysis of long-term fishery time series (1966-2013), this study highlights ...that an important driver of the presence of A. vulpinus in the northern Adriatic Sea, one of the most exploited basins of the Mediterranean, is the abundance of its prey, the sardine Sardina pilchardus. From 1997-2013, an annual average of 14 thresher sharks was landed at Chioggia (Italy), the main fish market in the area, mainly from July to October. More than 75% of the landed thresher sharks were sexually immature, with some specimens weighing less than 10 kg, indicating that a high proportion of the catch was composed of small young and newborn individuals. These results support the occurrence of a nursery area in the northern Adriatic Sea, highlighting the need for proper management of A. vulpinus catch in this area.Original Abstract: Zabiljezen je znacajan pad zastupljenosti psa lisice, Alopias vulpinus, u Sredozemnom moru. Analizom dugorocnog vremenskog niza podataka u ribarstvu (1966-2013), ovo istrazivanje pokazuje da je vazan pokretac prisutnosti A. vulpinus, u sjevernom Jadranu kao jednom od najiskoristenijih bazena na Sredozemlju, obilje njegova plijena srdele, Sardina pilchardus. U razdoblju 1997.-2013. godisnji prosjek ulova psine lisice je iznosio 14 primjeraka donesenih na glavnu ribarnicu u mjestu Chioggia (Italija) i to poglavito od srpnja do listopada. Vise od 75% primjeraka ulovljenog psa lisice bili su spolno nezreli, od kojih su neki primjerci imali manje od 10 kg, sto ukazuje da je visok udio ulova bio sastavljen od malih nedoraslih jedniki i mladji. Ovi rezultati podupiru pojavu rastilista u sjevernom Jadranu, ujedno naglasavajuci potrebu pravilnog upravljanja ulovom A. vulpinus u ovom podrucju.
PDF
